5.24.3 Input Line Traps

Set an input line trap. n is the number of lines of input
that may be read before springing the trap, macro is the macro to
be invoked. Request lines are not counted as input lines.

For example, one possible use is to have a macro that prints the next
n lines in a bold font.

.de B
. it \\$1 B-end
. ft B
..
.
.de B-end
. ft R
..

The itc request is identical except that an interrupted text line
(ending with \c) is not counted as a separate line.

Both requests are associated with the current environment
(see Environments); switching to another environment disables the
current input trap, and going back reactivates it, restoring the number
of already processed lines.